Google the Halo Map Version Converter and convert the maps back and forth from CE to PC and back. Only PC maps can be opened in HMT so the HMVC comes in handy.
I wouldn't recommend modding an existing CE map; if you do, it'll crash CE when you try to boot up CE. If not, then nobody can play your map except you.
Plus, modding an existing map is frowned upon by most modders.